Tarquin's 'guess' may well be the answer to this puzzle for these reasons:
a) the rock is said to heavy and hard - as is sarsen stone
b) The triangular section on the right in the second photo looks very much like silcrete
c) The small holes resemble the root trace fossils often seen in sarsens.
d) Sarsens are common in Wiltshire.
The shape is unusual for a sarsen fragment but I don't think it rules out the identification.